Glyphosate, sulfosate or fosametine are broad- spectrum systemic herbicides of the phosphonomethyl-glycine family. They act essentially as competitive inhibitors of 5-(enolpyruvyl)shikimate-3-phosphate synthase (EC 2.5.1.19) or EPSPS in relation to PEP (phosphoenolpyruvate). After their application to the plant, they are translocated inside the plant where they accumulate in the rapidly growing parts, in particular the caulinary and root apexes, causing the deterioration and even the destruction of sensitive plants.
Plastidial EPSPS, the main target of these products, is an enzyme of the aromatic amino acid biosynthesis pathway which is encoded by one or more nuclear genes and synthesised in the form of a cytoplasmic precursor and then imported into the plastids where it accumulates in its natural form.
The tolerance of plants to glyphosate and to products of the family is obtained by the stable introduction inside their genome of an EPSPS gene of plant or bacterial origin mutant or nonmutant with respect to the characteristics of the inhibition of the product of this gene by glyphosate. Given the mode of action of glyphosate, it is useful to be able to express the product of translation of this gene so as to permit its substantial accumulation in plastids.
It is known, for example from U.S. Pat. No. 4,535,060, to confer to a plant a tolerance to a herbicide of the abovementioned type, in particular N-(phosphonomethyl)glycine or glyphosate, by introducing into the plant genome a gene encoding an EPSPS carrying at least one mutation making this enzyme more resistant to its competitive inhibitor (glyphosate), after localisation of the enzyme in the plastidial compartment. However, these techniques need to be improved in order to achieve greater reliability in the use of these plants under agronomic conditions.
In the present description, "plant" is understood as meaning any differentiated multicellular organism capable of photosynthesis and "plant cell" any cell derived from a plant and capable of forming undifferentiated tissues such as calluses or differentiated tissues such as embryos or plant sections, plants or seeds.